Age-related functional changes of the glutamate receptor channels in rat Meynert neurones.

1. The developmental changes of glutamate receptors (GluRs) in acutely dissociated rat Meynert neurones were investigated using the conventional whole cell and nystatin perforated patch recording modes under voltage-clamp conditions. 2. The neurones became less responsive to N-methyl-D-aspartic acid (NMDA) with age, most dramatically between 1 day and 2 weeks, while the responses to kainic acid (KA) and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) gradually increased. The metabotropic GluR response appeared a few days after birth, but thereafter no further change was observed. 3. The decrease in the NMDA response during postnatal development was due to an abrupt reduction in the number of receptors without affecting the affinity, voltage-dependent Mg2+ blockade or high Ca2+ permeability (PCa/PCs approximately 7.0). 4. PCa/PCs in the presence of KA decreased from 2.8 in the 1-day-old (1D) rat neurones to 1.1 and 0.44 in the 2-week-old (2W) and 6-month-old (6M) rat neurones, respectively. The concentration-response relationship for KA shifted to the left with age. The KA response was not affected by NS-102, a KA-selective antagonist, thus indicating that the increased affinity of the receptor for the ligand resulted from the change in the AMPA receptor channel subunits. 5. The AMPA response in the presence of 10(-4) M cyclothiazide showed a change in the inward rectifying current-voltage relationship with age. The KA response was strongly cross-desensitized by the addition of AMPA and was also blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), whereas a rapid desensitization of the AMPA response was removed in a concentration-dependent manner by cyclothiazide. These results indicate that the non-NMDA receptor channels are assembled from the subunits of the AMPA receptor family without the GluR-2 subunit, thus resulting in a high Ca2+ permeability. 6. The L-glutamate (Glu)-induced responses were more sensitive to DL-2-amino-5-phosphonopentanoic acid (APV) in the 1D rat neurones than in the adult rat neurones. 7. Both NMDA and KA raised the intracellular Ca2+ concentration ([Ca2+]i) in all neurones of 1D, 2W and 6M rats, though the charybdotoxin-sensitive Ca(2+)-activated K+ current (IK(Ca)) did not appear in the 1D rat neurones. An age-related prolongation of both IK(Ca) decay and [Ca2+]i clearance was also seen after the removal of KA. 8. It was thus concluded that the age-related changes of ionotropic receptors appear to play a key role in the activities of immature and mature rat Meynert cholinergic neurones. The KA-induced IK(Ca), which developed with ageing, may thus function as one of the negative feedback systems, and thereby prevent excess cell excitation and neural damage, especially in adult rats.